objects: fix race in ObjectKillChildren() (fixes #13979)

If an object is created after the parent is already killed,
ObjectKillChildren() could not have killed the new object. So the new
object needs to inherit the alive flag from its parent instead.

src/misc/objects.c

@@ -133,7 +133,6 @@ void *vlc_custom_create (vlc_object_t *parent, size_t length,
     vlc_mutex_init (&priv->var_lock);
     vlc_cond_init (&priv->var_wait);
     priv->pipes[0] = priv->pipes[1] = -1;
-    atomic_init (&priv->alive, true);
     atomic_init (&priv->refs, 1);
     priv->pf_destructor = NULL;
     priv->prev = NULL;
@@ -157,6 +156,7 @@ void *vlc_custom_create (vlc_object_t *parent, size_t length,
 
         /* Attach the parent to its child (structure lock needed) */
         libvlc_lock (obj->p_libvlc);
+        atomic_init (&priv->alive, atomic_load (&papriv->alive));
         priv->next = papriv->first;
         if (priv->next != NULL)
             priv->next->prev = priv;
@@ -170,6 +170,7 @@ void *vlc_custom_create (vlc_object_t *parent, size_t length,
         obj->i_flags = 0;
         obj->p_libvlc = self;
         obj->p_parent = NULL;
+        atomic_init (&priv->alive, true);
         priv->next = NULL;
         vlc_mutex_init (&(libvlc_priv (self)->structure_lock));